This invention relates to improvements in an AC elevator control system for controlling an elevator car driven by an AC motor.
There are known AC elevator control systems of the type including an elevator car driven by an induction motor which is supplied with an AC electric power with a variable voltage at a variable frequency to have a controlled speed of rotation thereof. One of such AC elevator control system has comprised a three-phase electromagnetic switch and a converter serially connected to one another in the named order. The converter has converted a three-phase AC electric power from the AC power to a direct current and the output thereof has been connected across a smoothing capacitor for smoothing the direct current from the converter and also to an inverter including a plurality of transistors connected across respective semiconductor diodes with the reversed polarity and arranged in a three-phase bridge configuration to invert the smoothed direct current from the smoothing capacitor to a three-phase AC electric power with a variable voltage at a variable frequency. The three-phase AC electric power from the inverter has been supplied to a three-phase induction motor for driving an associated elevator car with the speed of rotation thereof controlled. The inverter has varied in voltage and frequency of the three-phase AC power in response to a command firing signal applied thereto from an inverter control device receiving an actual and a command speed signal for the elevator car.
During the stoppage of the elevator car, the electromagnetic switch has been maintained open to disable the smoothing capacitor to charge. Thus an electric charge on the capacitor has discharged due to reverse leakage currents through elements disposed in the converter and inverter, a current through an associated resistor for monitoring an overvoltage, etc. With the elevator car stopped for a long time interval, a voltage across the capacitor has sharply reduced. At that time the issue of a command start signal for the elevator car has caused the electromagnetic switch to be closed to permit a high charging current to flow into the smoothing capacitor. This has resulted in the disadvantages that contact pairs of the electromagnetic switch are welded due to their temperature rises, the capacitor reduces in lifetime due to a rush current therethrough, an overcurrent flows through the converter to break it, the circuit breaker malfunctions and so on.
Accordingly, it is an object of the present invention to provide a new and improved AC elevator control system including means for preventing a high charging current from flowing into a smoothing capacitor involved upon the start of an associated elevator car.